Many low cost, low grade petroleum and coal fuels contain appreciable levels of sodium, sulfur, and vanadium. Low quality marine residual fuels, for example, may contain up to 0.2 weight percent ash which contains appreciable amounts of sodium, up to 5 weight percent sulfur and up to 0.06 weight percent or 600 ppm vanadium. Low grades of marine distillate fuels are allowed up to 0.05 weight percent ash which contains appreciable amounts of sodium, up to 2 weight percent sulfur and up to 100 ppm vanadium. In addition, salts containing sodium, sulfur, and vanadium may also be ingested in the air intake of an engine or furnace operating in dusty or sea mist laden environments. Waste materials being burned in incinerators may, likewise, contain sodium, sulfur and vanadium.
In high temperature, oxygen-rich combustion environments, sodium, sulfur and vanadium are oxidized predominantly to the oxides of sodium, sulfur and vanadium, particularly sodium oxide, sulfur trioxide and vanadium pentoxide. These oxide species interact and condense as corrosive, molten deposits on such components as gas turbine blades, diesel engine exhaust valves and pistons, boiler tubes, and incinerator walls, or if gaseous, are found in the gas phase above. The corrosion caused by such deposits can be virulent and highly damaging, with the service life of the component being reduced by 80% or more, in some instances.
Molten deposits of this type are commonly described as "sodium sulfate-sodium vanadate" or "sulfate-vanadate" deposits. They contain sodium oxide, sulfur trioxide, sodium sulfate, sodium vanadate, and possibly other components, with sulfur trioxide gas in the gaseous phase above the deposits. Concentrations of the individual components in the molten and gaseous phases are fixed by such simultaneous chemical reactions as: EQU Na.sub.2 O+SO.sub.3 .revreaction.Na.sub.2 SO.sub.4 (sodium sulfate)[1] EQU Na.sub.2 O+V.sub.2 O.sub.5 .revreaction.2 NaVO.sub.3 (sodium metavanadate)[2] EQU 2 NaVO.sub.3 +SO.sub.3 .revreaction.Na.sub.2 SO.sub.4 +V.sub.2 O.sub.5 (sodium sulfate and vanadium pentoxide) [3]
The concentrations of the different components in the molten deposits and the gaseous phase thereabove depend also on the Na/V/S ratios in the fuel and intake air, as well as the temperature. Note that by reaction [3], an increase in the fuel sulfur content, for a fixed Na/V ratio, increases not only the Na.sub.2 SO.sub.4 concentration but also the V.sub.2 O.sub.5 concentration in the molten deposit.
Molten salt corrosion, or "hot corrosion", results when the oxides of sodium, sulfur and vanadium, such as Na.sub.2 O, SO.sub.3, and V.sub.2 O.sub.5 react with the protective surface oxide layer that exists on high temperature substrates, particularly metal alloys or ceramics. Under various circumstances, reaction with either sodium oxide, sulfur trioxide or vanadium pentoxide may be the predominant cause of hot corrosion. For example, in the well known "basic fluxing" mode of hot corrosion at 900.degree. C., Na.sub.2 O from Na.sub.2 SO.sub.4 -rich deposits reacts with the alumina surface film on gas turbine superalloys to form nonprotective NaAlO.sub.2 which causes catastrophic hot corrosion. High temperature chromium-containing metals which rely on chromia protective surface oxide are also corroded by Na.sub.2 SO.sub.4 -rich deposits via formation of nonprotective Na.sub.2 CrO.sub.4. Conversely, in 700.degree. C. "low temperature" hot corrosion, sulfur trioxide gas, which is present in the corrosive environment, reacts with cobalt oxide from CoCrAlY turbine blade coatings to form low-melting mixed CoSO.sub.4 -Na.sub.2 SO.sub.4 eutectic deposits and yields an "acidic fluxing" type of hot corrosion. Fuels containing vanadium often cause a "vanadate" type of hot corrosion wherein vanadium pentoxide from the molten deposit reacts with nickel oxide or cobalt oxide from nickel-based or cobalt-based superalloys, or with ferric oxide from high temperature steels, to produce spalling metal vanadates.
Similar hot corrosion reactions involving sodium oxide, sulfur trioxide, and vanadium pentoxide occur with ceramics. For instance, sodium oxide from Na.sub.2 SO.sub.4 -rich molten deposits reacts with the surface film of silica that normally protects silicon carbide and silicon nitride ceramics in oxygen-rich engine environments. This type of hot corrosion represents a serious threat to the use of silicon carbide and silicon nitride components in land and marine engines. High sulfur fuels, containing in excess of 1 weight percent sulfur, attack yttria-stabilized zirconia thermal barrier coatings through reaction of the engine gas sulfur trioxide with yttria to form low-melting eutectic sulfate mixtures. The reaction of yttria in yttria-stabilized zirconia with the vanadium pentoxide component of sulfate-vanadate engine deposits is another ceramic hot corrosion problem which, at present, precludes use of yttria-stabilized zirconia thermal barrier coatings in marine and land engines burning fuels which contain vanadium.
Therefore, no material is known which inhibits hot corrosion in the presence of vanadate-sulfate molten deposits containing high concentrations of oxides of sodium and vanadium in the melt and gaseous oxides of sulfur in the gaseous phase thereabove.